I just made a clean install of Ubuntu 11.10, downloaded and installed Oracle Java JDK and JRE, version 7, and downloaded Eclipse Juno from eclipse.org and unpacked it to a folder. I started Eclipse, imported a preferece file, and then opened an exsiting workspace.

When Content Assist appears the background is black which is a bit strange but the white text is readable, what is unreadable is the dark blue text which details any sugggested options.

I've made preference changes to make the background yellow but nothing changes.

There are a couple of things you can do. The one I use is System ->
Preferences -> Appearances -> Theme -> Clearlooks. Obviously, this has a
radical effect on the rest of your desktop theme experience, but it's
perfect for what you're trying to solve.